Magnetic recording media generally include a binder dispersion layer comprising a binder and a pigment overlying a substrate, wherein the pigment is dispersed within the binder. Typically, the pigment is a magnetizable pigment comprising small, magnetizable particles. In some instances, the medium may be in the form of a composite having both back-coat and front-coat binder dispersion layers, although the pigment in the back-coat may or may not be a magnetizable pigment.
It has become desirable to have as high a loading of magnetizable pigment in the magnetic recording media as is reasonably possible. It is often preferred to have a binder dispersion comprising from about 70% to 85% by weight magnetizable pigment relative to the binder with as many magnetizable particles per unit area or unit volume as possible. It is also preferred to have a binder dispersion in which the magnetizable pigment comprises a plurality of small particles having a relatively high specific surface area. Higher pigment loading has the potential to provide high density magnetic recording media capable of storing more information.
Problems, however, remain in the art concerning magnetic recording media having a relatively high loading of magnetizable pigment. To begin with, magnetizable pigments tend to agglomerate, and they are difficult to properly and fully disperse within the binder. Wetting agents, or dispersants, are often employed to facilitate such dispersion. For higher pigment loading, i.e., the use of greater amounts by weight and number of magnetizable particles, greater amounts of such dispersants are required, which is not always desirable.
There are a number of reasons for using as little dispersant as possible. Dispersants tend to soften binder systems, decrease their toughness, and adversely affect their cure. Binders without dispersants can be more readily and reproducibly prepared. Further, excess dispersant may bloom from a cured binder system over time, leading to contamination of a recording head or the like, causing a change in the physical or chemical characteristics of the media.
To help alleviate these problems with added dispersants, binder compositions having internal (chemically bound) dispersants have been developed. Such compositions comprise polymers with functional moieties pendant from the polymer backbone that help disperse the magnetizable pigment. As a result of using these compositions, less dispersant or, in some instances, no dispersant is needed for dispersion of magnetic pigment in the binder. However, in spite of these improvements, a higher degree of pigment-polymer interaction is desired, particularly in the case of pigments exhibiting small particle size, such as barium ferrite.
Recently, sulfonated hydroxy-functional polyurethane-containing binder systems have been described which exhibit excellent pigment loading and magnetic orientation (U.S. Pat. Nos. 5,071,578 and 5,085,941, both assigned to 3M, both incorporated by reference herein). However, these sulfonated polyurethane binder systems are less effective in dispersing barium ferrite (BaFe) pigments, a class of pigments of increasingly important commercial consideration.
Polyurethane magnetic binder polymers containing isolated carboxylic acid groups which do not have additional polar functional groups associated therewith (i.e., are not capable of chelation) are known in the art. "Chelating agents" (materials capable of chelation) refers to materials which have two or more atoms which may serve as electronegative donors positioned so that the donor atoms may react with a metal atom, metal ion, or metal surface to form a five-membered or six-membered ring structure. Donor atoms are selected from oxygen, nitrogen and sulfur. Donor groups include alcohols, enols, phenols, ethers, carboxylic acids and carbonyl groups such as aldehydes, ketones, carboxylic esters and carboxamides, and their thio-analogs. Nitrogen-containing donor groups include imines and amines and moieties in which the nitrogen atom forms part of a heterocyclic ring. Such materials with two or more donor groups are also known in the art as "bidentate," "polydentate," or "multidentate," and these terms, when used herein, are used interchangeably.
U.S. Pat. No. 4,612,244 (to Sony Corp.) discloses the potential use of a metal salt of hydroxyacetic acid as a component of a polyurethane binder for a backside coating of magnetic recording tape. U.S. Pat. No. 4,613,345 (to Sony Corp.) discloses the possible use of chloroacetic acid to produce carboxyl-functional magnetic binders. U.S. Pat. No. 4,571,364 (to Fuji Photo Film Co.) discloses polyurethane resins for magnetic binders in which lysine (or its salts) or 2-alkyl-2-carboxy-1,3-propanediols are incorporated into the polymer. None of these discloses or claims the use of multidentate carboxyl-functional polyurethanes. U.S. Pat. No. 4,788,103 (to Fuji Photo Film Co.) describes a magnetic pigment binder comprising a polyurethane containing a polar group such as a carboxylic acid or a carboxylic acid salt. Barium ferrite particles are among those disclosed, but not claimed, as compatible with the binder resin.
A carboxyl-functional polyurethane resin for magnetic media is commercially available from Sanyo Chemical Industries under the trade name TI7503. Japanese Patent Applications JP 03 64,310 and JP 03 64,314, both to Sanyo, describe a urethane binder prepared by using, inter alia, dimethylolpropionic acid or its ammonium salt. The binders disclosed in the two Sanyo applications have only isolated carboxylic acid groups.
U.S. Pat. No. 4,096,127, to Akzona, discloses preparation of carboxyl-functional polyurethanes by half-esterification of hydroxy groups pendant from the polyurethane backbone with aliphatic or aromatic dicarboxylic acid anhydrides. Acid anhydrides of the invention have no additional functionality, and the resultant ester-acids are converted to salts and used as paper sizing agents. No applications towards magnetic media are taught.
Hydroxylated polycarboxylic acids of low molecular weight, such as citric acid, tartaric acid or 1-malic acid have been used as non-chemically-bonded additives in magnetic media articles. U.S. Pat. No. 4,693,930, to 3M, discloses the use of citric acid as a lubricant-adsorption inhibitor. Similarly, U.S. Pat. No. 5,066,539, to Sony Corporation, discloses the use of citric acid and nitrilotriacetic acid as non-reactive additives to magnetic media preparations.
Chelating agents which are included in polymer systems are also described in U.S. Pat. No. 5,026,860, assigned to the assignee of the present invention, which is incorporated by reference herein. Ethylenically-unsaturated copolymerizable chelating monomers are described. Non-copolymerizable arthydrides are not described.
Blends of polyurethanes with vinyl chloride resins are known in the magnetic pigment binder patent art. Vinyl chloride resins containing polar functionality, especially sulfonate or ammonium salts, are capable of producing good dispersions by themselves, but these resins generally lack sufficient toughness and flexibility for most product applications. Blending of these vinyl chloride resins with polyurethane resins improves toughness of the resulting binder but care must be taken to preserve dispersion quality. Commercially available carboxyl-functional polyurethanes are compatible with ammonium vinyl chloride resins but the known carboxyl-functional polyurethanes interact weakly with pigments and produce relatively poor dispersions by themselves and, in general, they degrade the dispersing capability of ammonium vinyl chloride resins when blended therewith.
A need thus exists for a polyurethane binder composition which is capable of irreversible binding of pigment particles and which produces smooth, high loading barium ferrite dispersions which provide pigment coatings with low void volume and excellent magnetic and mechanical properties, and which are compatible with, e.g., ammonium vinyl chloride resins.